[2/n] Replace read-only desktop windowing flag with runtime flag

Replace use of `enable_desktop_windowing` flag with
`enable_desktop_windowing_mode` flag

Flag: NONE
Bug: 304778354
Fixes: 326060018
Test: build & `adb shell device_config put lse_desktop_experience
com.android.window.flags.enable_desktop_windowing_mode true && adb
reboot` desktop windowing mode should be enabled.

Change-Id: Idc44e2b7a8b738f15f8d3812a0c4fc1a4b4861c1